1. Field of the Invention
The present invention relates in general to the field of signal processing in communication systems, and more particularly to a decision sequence generating method that can enable an associated receiver containing a decision feedback equalizer (DFE) to generate a decision based on a whole codeword to replace chip decisions generated previously, thereby reconstructing post-cursor section of channel impulse response more precisely.
2. Description of the Prior Arts
Wireless local area network (WLAN) is one of the most popular and fast-developed technologies, and the new WLAN-related specifications and standards are proposed quickly. However, The application of WLANs for high-speed and reliable wireless communication has many problems to overcome, one critical issue of these problems is multi-path distortion. Because there exists various transmission paths in the channel between the transmitter and the receiver in a wireless communication system, inter-symbol interference (ISI) and exponentially decayed multi-path effect of transmitted signals received by the receiver are generated.
In WLAN systems, data is transmitted in the form of codewords. Each codeword is composed of N chips, and N is an integer. The total number of the codewords used in a codeword set and the chip number N of a codeword are determined by the specific operation mode of communication which the system employs. The multi-path distortion mentioned above includes inter-symbol interference (ISI) and inter-chip interference (ICI, or called intra-symbol interference).
FIG. 1 shows a multi-path channel impulse response characteristic 10. As shown in FIG. 1, we can divide the channel impulse response into three sections: (1) peak section 11, which has the maximum energy that we want to reserve; (2) pre-cursor section 12, which appears before the peak 11 and usually has a shorter length; and (3) post-cursor section 13, which appears after the peak 11 and usually has a long decay “tail” on the impulse response characteristic 10, wherein the pre-cursor section 12 and post-cursor section 13 are the main cause of the ISI and ICI.
Typically, a DFE is applied in a receiver of a wireless communication system to remove these “non-ideal” sections of the channel impulse response, and then ICI and ISI generated by transmission through the multi-path channel would be mitigated effectively.
FIG. 2 shows a block diagram of a conventional DFE. As shown in FIG. 2, DFE 20 comprises: a feed-forward filter 21, an adder 22, a decision unit 23 and a feedback filter 24, wherein the feed-forward filter 21 and the feedback filter 24 are used to remove the pre-cursor section 12 and the post-cursor section 13 respectively.
The signal processing flow for the DFE 20 comprises the following steps:
(1) Provide a chip received by the receiver to the feed-forward filter 21 to remove the effect caused by the pre-cursor section 12 from the chip.
(2) Send the chip into the adder 22 and subtract the re-constructed post-cursor section 13 from the feedback filter 24. The feedback filter 24 is comprised of a tap-delay line and well-trained post-cursor channel coefficients.
(3) Functionally, the DFE 20 is used to remove the part of the inter-symbol and intra-symbol interference, which is estimated based on previously detected symbols, from the current chip. The corrected chip is then sent into the decision unit 23.
(4) At last, the decision unit 23, which contains a limiter, determines a chip decision for the corrected chip, and then sends it to the feedback filter 24 and other devices for subsequent processing.
However, the operation of the conventional DFE 20 is chip-based and needs a higher signal-to-noise ratio (SNR) environment to operate effectively. If SNR is not high enough, incorrect decisions will be made by the decision unit 23, and then propagate down the feedback filter 24. It's very probable to impact the accuracy of all subsequent chip decisions because of only a few wrong chip decisions.
In order to mitigate the serious error-propagation problem mentioned, the present invention provides a decision sequence generating method and an associated receiver with a DFE. Besides operating in chip-based way, the decision sequence generating method can enable the DFE to generate a decision based on a whole codeword to replace chip decisions provided to a feedback filter of the DFE previously, thereby reconstructing the post-cursor section of channel impulse response with more confidence.